Air, land, and water all receive emissions from landfills. The time frame for regulatory concern is at least 30 years after the landfill is closed. Air emission rates slow after 840 years (Augenstein and Pacey, 1991), but some air emissions and leachate production can be expected to continue for a century or more.
MSW Landfills
Air Emissions
Approximately 60-110 pounds of methane per ton of wet MSW will be formed in the landfill during the first 20 years. About W16 pounds of that gas will not be recovered, but will leak because of the limitations of the collection system and the permeability of the cover (ERR, l991f) into the atmosphere within that same period with at most a short delay(6). The EPA estimates that 12 million tons per year of methane are released from U.S. landfills (FR, l991b).
Environmental releases from landfills consist of uncaptured emissions of trace amounts of a variety of hazardous gases, as well as larger quantities of methane and of CO2, which is generated in the landfill in volumes approximately equal to that for methane. These emissions occur both through leakage and through separation from the captured landfill gases. In addition, the EPA estimates that present landfills release 283,000 tons per year of nonmethane organic compounds (NMOCs)-or about 1% of U.S. stationary source emissions (FR,1991b). Table 6.2 presents analyses of landfill gases.
New proposed regulations of air emissions from landfills (FR, 1991f) will increase the number of landfills that actively recover gas. As mentioned, 157 landfills already operate or are planning to operate landfill-gas-to-energy projects, and about 87 of the 930 new landfills that are projected to begin operations between 1992 and 1997 will include gas recovery in response to the new regulations. The EPA predicts that larger landfills expected to remain in operation over the next 10 years will also add gas recovery operations as a result of the regulations (FR,1991f).
By regulation, leachate must be captured and treated because it can contaminate ground-water. If leachate is treated by spraying or recirculated by spraying it on the working face, some of the volatile organic materials it contains are likely to be released intact to the atmosphere. If the leachate is treated in a sewage treatment plant, the normal first step is to aerate the waste, and many of the organic materials may be volatilized at the point, without being decomposed. No estimates of these releases were found; data on releases derived from leachate treatment are needed.
Water Emissions
For approximately 30 years after closure, leachate must be captured and treated, and nearby groundwater must be monitored. After 30 years, monitoring can stop if measured concentrations of specified pollutants are found to be less than regulated limits (FR,1991q). However, leaching caused by the infiltration of rainwater will continue.
The EPA has developed a computer model (Hydrologic Evaluation of Landfill Performance-HELP) to predict the amount of rain that will run off or evaporate from the cover of the landfill. This model also estimates the amount that will enter the landfill, as well as the proportion that will leak through the bottom liner into the ground below (O'Leary and Walsh, 1991).
Table 6.2 shows the amounts of materials that could leach over 20 years (the time span used in this study) into the portion of leachate that is collected for treatment, as well as the portion that would pass through the liner into the ground below. These data are based on concentrations reported by O'Leary and Walsh, (1991) and on landfill volume and area requirement data for Will County, Illinios (Patrick Engineering, 1991), together with estimates of average concentrations of leachate from an MSW landfill. The information is based on limited data, particularly on the range of concentrations of metals in the leachate over long periods, and it needs to be supplemented by other studies to provide a realistic range.
Land Use
Typical landfills are 50 feet deep with a density of 50,000 tons per acre. Larger landfills can be as deep as 100 to 250 feet and can have capacities of more than 10 million tons (FR,1991p). After a landfill reaches design capacity, it is covered with compacted clay to prevent the infiltration of water. Because federal and state regulations require gas control, such systems are often installed as sections of the landfill are completed. After a landfill is closed, restricted uses of the land over the landfill (e.g., for parks, recreational facilities) can begin almost immediately. Because of settlement and possible gas leakage, some sources have estimated that 30 to 50 years will be needed before unrestricted use of the land (e.g., housing, industrial and commercial facilities) will be possible (Vesilind et al., in press). In general, closed landfills are most suitable for growing grasses and similar plants with shallow root systems. Special care is required for growing trees. Buildings may be installed in areas where land values are high, but special construction techniques are required (Walsh, 1992).
Ash Monofills
If MSW is burned instead of landfilled, the ash from combustion is normally landfilled. The ash can range from about 17% of the weight of refuse-derived fuel (RDF) that is burned to about 24% of the weight of MSW burned in a mass combustion plant. Because the density of the ash is much higher than the density of compacted landfill, the space required for the ash amounts to about 10% of the space the original MSW would require (FR,1991a; also calculated from Patrick Engineering, 1991). Some constituents of the ash are shown in Table 6.3.
Current landfill regulations make no distinction between construction and operation of landfills for MSW or for ash (CFR, 1991c). In practice, ash is often disposed of in landfills that accept ash only (called ash monofills) because the metals in the ash leach more readily under acid conditions, and one phase of a normal landfill decomposition reaction of MSW creates acids. In a monofill, no acids are generated, and metal dissolution is retarded.
Table 6.2
AMOUNTS OF MATERIAL LEACHING FROM AN MSW LANDFILL
Cumulative Leachate Quantity(a)
Concentration(b) Captured for Treatment, Escaping Through Liner, Escape Through Liner,
20 years(c) 20 years(c) 20 years(c)
Compound/
Element Mean Range Mean Range Mean Range Mean Range Mean Range
Milligrams per Liter Grams per Ton Grams per Ton Grams per Ton Pounds per Ton
Cl 2100 100-5000 434 20.6-1030 81.1 3.9-193 515 24.5-1226 1.13 0.05-2.7
Na 1350 50-4000 279 10.3-826 52.1 1.9-154 331 12.2-980 0.73 0.02-2.16
K 1100 10-2500 227 2.1-516 42.5 0.4-96 270 2.5-612 0.59 0.005-1.34
Micrograms per Liter Milligrams per Ton Milligrams per Ton Milligrams per Ton 10(-6) Pounds per Ton
AOX(d) 2000 350-3500 413 72.3-723 77.2 12.2-135 490 78.5-858 1080 173-1842
As 160 5-1600 33 1.0-330 6.2 0.2-61.8 39.2 1.2-392 86.4 2.6-864
Cd 6 0.5-140 1.2 0.1-28.9 0.2 0.02-5.4 1.4 0.1-34.3 3.08 0.22-75.6
Co 55 4-950 11.4 0.8-196 2.1 0.2-36.7 13.5 1.0-233 29.7 2.2-514
Ni 200 20-2050 41.3 4.1-423 7.7 0.8-79.1 49.0 4.9-502 108 10.8-1107
Pb 90 8-1020 18.5 1.6-210 3.5 0.3-39.4 22.0 1.9-250 48 4.2-551
Cr 300 30-1600 62 6.2-330 11.6 1.2-61.8 73.6 7.4-392 162 16-864
Cu 80 4-1400 16.5 0.8-289 3.1 0.2-54.0 19.6 1.0-343 43.2 2.2-26.9
Hg 10 0.2-50 2.1 0.04-10.3 0.4 0.01-1.9 2.5 1.9-12.2
COD 3000 500-4500 619 - 116 - 734 -
(a) These estimates probably represent the largest possible emissions of heavy metals; a lower value was used in the data base.
(b) Source: O'Leary and Walsh, 1991
(c) Leaching will continue after 20 years.
(d) AOX = absorbable organic halogen.
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Table 6.3
SOME CONSTITUENTS OF COMBINED MSW ASH
Parts per Million Pounds per Ton Pounds per Ton
in Ash in Ash of MSW
Inorganic Constituents
Aluminum 5,000-60,000 10-120 2.5-30
Calcium 4,100-85,000 8.2-170 2.05-42.5
Iron 690-133,500 1.38-267 0.34-66.8
Lead 31Ñ36,600 0.062-73.2 0.155-18.3
Magnesium 700-16,000 1.4-32 0.35-8.0
Nickel 13-12,900 0.026-25.8 0.0065-6.45
Potassium 290-12,000 0.58-24 0.145-6.0
Titanium 1,000-28,000 2.0-56 0.5-14.0
Zinc 92-46,000 0.184-92 0.046-23
Barium 79-2,700 0.158-5.4 0.04-1.35
Chromium 12-1,500 0.024-3.0 0.006-0.75
Cobalt 1.7-91 0.0034-0.182 0.00085-0.046
Copper 40-5,900 0.080-11.8 0.02-2.95
Manganese- 14-3,130 0.028-6.26 0.007-1.56
Phosphorus 290-5,000 0.58-10 0.145-2.5
Antimony <120-<260 <0.24<0.52 <0.06-<0.13
Boron 24-174 0.0048-0.0348 0.012-0.087
Cadmium 0.18-100 0.00036-0.20 0.00009-0.05
Molybdenum 2.4-290 0.0048-0.58 0.0012-0.145
Tin 13-380 0.026-0.76 0.0065-0.19
Vanadium 13-150 0.026-0.30 0.0065-0.075
Arsenic 2.9-50 0.0058-0.10 0.0014-0.25
Mercury 0.05-17.5 0.0001-0.035 0.000025-0.0087
Lithium 6.9-37 0.0138-0.074 0.003-0.018
Selenium 0.10-50 0.0002-0.0002 0.00005-0.025
Silver 0.05-17.5 0.0001-0.035 0.000025-0.0087
Yttrium 0.55-8.3 0.0011-0.0166 0.00028-0.004
Beryllium ND-2.4 ND-0.0012
Organic Constituents
PCDD 0.0062-0.350 0.0000124-0.0007 0.0000031-0.000175
PCDF 0.00614-0.154 0.0000123-0.000308 0.0000031-0.000077
PCB ND-0.0322 ND-0.0000644 ND-0.0000161
Source: Repa and Kiser, 1988
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The Clean Air Act Amendments of 1990 (424SC 6921, Section 306) Section gives EPA until 1993 to decide whether MSW ash is hazardous or nonhazardous. California and Massachusetts have declared the ash to be nonhazardous. Illinois applies a Toxic Characteristic Leaching Procedure (TCLP) test, which is an acidic leach procedure that the ash normally fails to pass; ash that fails the test is treated as a hazardous waste.
The current EPA regulations (CFR, l991c) require:
No studies on air emissions from ash monofills were located. Air emissions from monofills will be lower than those from MSW landfills because ash contains very low concentrations of biodegradable organics, or none at all.
Other possible air emissions could result: (1) from organic materials adsorbed on the carbon that occurs in flyash, particularly RDF flyash; and (2) possibly from mercury that is initially absorbed on the flyash, the scrubber lime, or the carbon. The opinion is becoming more prevalent that the carbon in flyash actively adsorbs organic materials and metals from flue gas (ICF, 1991).
Although more than 30 species or organic materials, dioxins, furans, and polychlorinated biphenyls (PCBs) have been detected at low levels in flyash, as shown in Table 6.3, they are not particularly volatile and adsorb strongly to materials Cones, 1991; Rigo, 1991).
Although mercury has been reported to evaporate from ash samples collected from MSW baghouses (in Germany) while the samples were stored in laboratories, no reports on metal emissions from ash monofills were found (Bergstrom, 1986).
Water Emissions
Overall, for each single ton of original MSW at the curb, the leachate from ash monofills appears to be much smaller in volume than that from normal landfills. For landfills of equal depth, the difference is a consequence of the smaller volume and surface area occupied by the ash monofill; the smaller area receives less rain that could become leachate.
Because of its alkalinity, the leachate from an ash monofill also appears to have lower metal concentrations than the leachate from a raw MSW landfill. Combustors that use lime for acid-gas control create a residue that causes the ash to harden; the lime apparently further reduces the leaching potential of the ash (Varello, 1992). However, these conclusions are based on extremely limited data, and no ash monofills have been monitored over a long period, although such studies are under way (Roffman, 1992). No study was found Chat considered She effect of hardening of the ash on the amount of leachate Chat passes Through an ash monofill and is either captured in a collection system or escapes through the liner.
Ash derived from burning MSW contains virtually all the metals that were originally present in the waste. The low levels of organic materials present in the ash could dissolve in the leachate. The composition of the leachate resulting from rain on a monofill is shown in Table 6.4. The single study of ash monofills on which Tables 6.3 and 6.4 are based found that levels of metals and organic materials in the ash were extremely low Hoffman, 1992).
TABLE 6.4
LEACHATE FROM AN ASH MONOFILL
Leachate
Concentration(a) Cumulative Release into Leachate over 20 Years: Range
Parts per Million Grams per Ton Grams per Ton Pounds per Ton
(Milligrams per Liter) of Ash(b) of MSW of MSW
Chloride 7,700 (88c)-30,700 (91) 189-752 48-190 0.49-2.13
Sodium 3,000 (89)-6,340 (90) 74-1290 19-325 0.19-0.40
Potassium 516 (89)-4,320 (90) 72-102 18-26 0.03-0.27
Parts per Billion Milligrams per Ton Milligrams per Ton 10(-6) Pounds per
(Micrograms per Liter) of Ash of MSW Ton of MSW
Arsenic ND (91)-260 (88) ND-6.4 ND-1.6 ND-25.4
Cadmium ND (88,90,91)-1.4 (89) ND->0.1 ND->0.1 ND-0.11
Chromium ND (89,90,91)-32 (89) ND-0.8 ND-0.2 ND-2.03
Copper ND-ND ND-ND ND-ND ND-ND
Nickel ND-ND ND-ND ND-ND ND-ND
Lead ND (91)-54 (89) ND-0.7 ND-0.2 ND-3.40
Mercury ND-ND ND-ND ND-ND ND-ND
Zinc ND (91)-370 (88) ND-9 ND-2.3 ND-23.5
(a) Source: Roffmann, 1991.
(b) Projected from Roffmann's data, as described in the subsection entitled "Missing Data."
(c) Numbers in parenthese indicate the year the concentrations were measured.
were measured.
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Land Use
No data were found on whether restrictions on land use are necessary after an ash monofill is closed. Because of the density of the monofill and the lack of gas emissions, fewer restrictions on land use would probably be necessary for closed ash monofills than for closed MSW landfills. At present, however, few ash monofills have been closed, and only a small number of them seem to be candidates for development (Walsh, 1992).
Assumptions about the beneficial uses of stabilized ash are frequently based on relatively extensive research on the uses for flyash from coal-fired utilities. Some studies have evaluated ash as a component of bituminous highway material. Such use would reduce the amount that needed to be landfilled. Other research is under way on its use in masonry block construction materials. Some processes vitrify or melt the ash into a glass that is extremely inert to leaching and can often be used beneficially as aggregate; see Appendix A and DeCesare (1991).
Integrated Strategy Example: MSW Collection and Landfill
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